@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2007, 2008, 2009, 2010
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Modules
@section Modules
@cindex modules

When programs become large, naming conflicts can occur when a function
or global variable defined in one file has the same name as a function
or global variable in another file.  Even just a @emph{similarity}
between function names can cause hard-to-find bugs, since a programmer
might type the wrong function name.

The approach used to tackle this problem is called @emph{information
encapsulation}, which consists of packaging functional units into a
given name space that is clearly separated from other name spaces.
@cindex encapsulation
@cindex information encapsulation
@cindex name space

The language features that allow this are usually called @emph{the
module system} because programs are broken up into modules that are
compiled separately (or loaded separately in an interpreter).

Older languages, like C, have limited support for name space
manipulation and protection.  In C a variable or function is public by
default, and can be made local to a module with the @code{static}
keyword.  But you cannot reference public variables and functions from
another module with different names.

More advanced module systems have become a common feature in recently
designed languages: ML, Python, Perl, and Modula 3 all allow the
@emph{renaming} of objects from a foreign module, so they will not
clutter the global name space.
@cindex name space - private

In addition, Guile offers variables as first-class objects.  They can
be used for interacting with the module system.

@menu
* General Information about Modules::  Guile module basics.
* Using Guile Modules::         How to use existing modules.
* Creating Guile Modules::      How to package your code into modules.
* Module System Reflection::    Accessing module objects at run-time.
* Included Guile Modules::      Which modules come with Guile?
* R6RS Version References::     Using version numbers with modules.
* R6RS Libraries::              The library and import forms.
* Accessing Modules from C::    How to work with modules with C code.
* Variables::                   First-class variables.
* provide and require::         The SLIB feature mechanism.
* Environments::                R5RS top-level environments.
@end menu

@node General Information about Modules
@subsection General Information about Modules

A Guile module can be thought of as a collection of named procedures,
variables and macros.  More precisely, it is a set of @dfn{bindings}
of symbols (names) to Scheme objects.

An environment is a mapping from identifiers (or symbols) to locations,
i.e., a set of bindings.
There are top-level environments and lexical environments.
The environment in which a lambda is executed is remembered as part of its
definition.

Within a module, all bindings are visible.  Certain bindings
can be declared @dfn{public}, in which case they are added to the
module's so-called @dfn{export list}; this set of public bindings is
called the module's @dfn{public interface} (@pxref{Creating Guile
Modules}).

A client module @dfn{uses} a providing module's bindings by either
accessing the providing module's public interface, or by building a
custom interface (and then accessing that).  In a custom interface, the
client module can @dfn{select} which bindings to access and can also
algorithmically @dfn{rename} bindings.  In contrast, when using the
providing module's public interface, the entire export list is available
without renaming (@pxref{Using Guile Modules}).

To use a module, it must be found and loaded.  All Guile modules have a
unique @dfn{module name}, which is a list of one or more symbols.
Examples are @code{(ice-9 popen)} or @code{(srfi srfi-11)}.  When Guile
searches for the code of a module, it constructs the name of the file to
load by concatenating the name elements with slashes between the
elements and appending a number of file name extensions from the list
@code{%load-extensions} (@pxref{Loading}).  The resulting file name is
then searched in all directories in the variable @code{%load-path}
(@pxref{Build Config}).  For example, the @code{(ice-9 popen)} module
would result in the filename @code{ice-9/popen.scm} and searched in the
installation directories of Guile and in all other directories in the
load path.

A slightly different search mechanism is used when a client module
specifies a version reference as part of a request to load a module
(@pxref{R6RS Version References}).  Instead of searching the directories
in the load path for a single filename, Guile uses the elements of the 
version reference to locate matching, numbered subdirectories of a 
constructed base path.  For example, a request for the 
@code{(rnrs base)} module with version reference @code{(6)} would cause
Guile to discover the @code{rnrs/6} subdirectory (if it exists in any of
the directories in the load path) and search its contents for the
filename @code{base.scm}.

When multiple modules are found that match a version reference, Guile
sorts these modules by version number, followed by the length of their
version specifications, in order to choose a ``best'' match.

@c FIXME::martin:  Not sure about this, maybe someone knows better?
Every module has a so-called syntax transformer associated with it.
This is a procedure which performs all syntax transformation for the
time the module is read in and evaluated.  When working with modules,
you can manipulate the current syntax transformer using the
@code{use-syntax} syntactic form or the @code{#:use-syntax} module
definition option (@pxref{Creating Guile Modules}).


@node Using Guile Modules
@subsection Using Guile Modules

To use a Guile module is to access either its public interface or a
custom interface (@pxref{General Information about Modules}).  Both
types of access are handled by the syntactic form @code{use-modules},
which accepts one or more interface specifications and, upon evaluation,
arranges for those interfaces to be available to the current module.
This process may include locating and loading code for a given module if
that code has not yet been loaded, following @code{%load-path} (@pxref{Build
Config}).

An @dfn{interface specification} has one of two forms.  The first
variation is simply to name the module, in which case its public
interface is the one accessed.  For example:

@lisp
(use-modules (ice-9 popen))
@end lisp

Here, the interface specification is @code{(ice-9 popen)}, and the
result is that the current module now has access to @code{open-pipe},
@code{close-pipe}, @code{open-input-pipe}, and so on (@pxref{Included
Guile Modules}).

Note in the previous example that if the current module had already
defined @code{open-pipe}, that definition would be overwritten by the
definition in @code{(ice-9 popen)}.  For this reason (and others), there
is a second variation of interface specification that not only names a
module to be accessed, but also selects bindings from it and renames
them to suit the current module's needs.  For example:

@cindex binding renamer
@lisp
(use-modules ((ice-9 popen)
              #:select ((open-pipe . pipe-open) close-pipe)
              #:renamer (symbol-prefix-proc 'unixy:)))
@end lisp

Here, the interface specification is more complex than before, and the
result is that a custom interface with only two bindings is created and
subsequently accessed by the current module.  The mapping of old to new
names is as follows:

@c Use `smallexample' since `table' is ugly.  --ttn
@smallexample
(ice-9 popen) sees:             current module sees:
open-pipe                       unixy:pipe-open
close-pipe                      unixy:close-pipe
@end smallexample

This example also shows how to use the convenience procedure
@code{symbol-prefix-proc}.

You can also directly refer to bindings in a module by using the
@code{@@} syntax.  For example, instead of using the
@code{use-modules} statement from above and writing
@code{unixy:pipe-open} to refer to the @code{pipe-open} from the
@code{(ice-9 popen)}, you could also write @code{(@@ (ice-9 popen)
open-pipe)}.  Thus an alternative to the complete @code{use-modules}
statement would be

@lisp
(define unixy:pipe-open (@@ (ice-9 popen) open-pipe))
(define unixy:close-pipe (@@ (ice-9 popen) close-pipe))
@end lisp

There is also @code{@@@@}, which can be used like @code{@@}, but does
not check whether the variable that is being accessed is actually
exported.  Thus, @code{@@@@} can be thought of as the impolite version
of @code{@@} and should only be used as a last resort or for
debugging, for example.

Note that just as with a @code{use-modules} statement, any module that
has not yet been loaded yet will be loaded when referenced by a
@code{@@} or @code{@@@@} form.

You can also use the @code{@@} and @code{@@@@} syntaxes as the target
of a @code{set!} when the binding refers to a variable.

@c begin (scm-doc-string "boot-9.scm" "symbol-prefix-proc")
@deffn {Scheme Procedure} symbol-prefix-proc prefix-sym
Return a procedure that prefixes its arg (a symbol) with
@var{prefix-sym}.
@c Insert gratuitous C++ slam here.  --ttn
@end deffn

@c begin (scm-doc-string "boot-9.scm" "use-modules")
@deffn syntax use-modules spec @dots{}
Resolve each interface specification @var{spec} into an interface and
arrange for these to be accessible by the current module.  The return
value is unspecified.

@var{spec} can be a list of symbols, in which case it names a module
whose public interface is found and used.

@var{spec} can also be of the form:

@cindex binding renamer
@lisp
 (MODULE-NAME [:select SELECTION] [:renamer RENAMER])
@end lisp

in which case a custom interface is newly created and used.
@var{module-name} is a list of symbols, as above; @var{selection} is a
list of selection-specs; and @var{renamer} is a procedure that takes a
symbol and returns its new name.  A selection-spec is either a symbol or
a pair of symbols @code{(ORIG . SEEN)}, where @var{orig} is the name in
the used module and @var{seen} is the name in the using module.  Note
that @var{seen} is also passed through @var{renamer}.

The @code{:select} and @code{:renamer} clauses are optional.  If both are
omitted, the returned interface has no bindings.  If the @code{:select}
clause is omitted, @var{renamer} operates on the used module's public
interface.

In addition to the above, @var{spec} can also include a @code{:version} 
clause, of the form:

@lisp
 :version VERSION-SPEC
@end lisp

where @var{version-spec} is an R6RS-compatible version reference.  The 
presence of this clause changes Guile's search behavior as described in
the section on module name resolution 
(@pxref{General Information about Modules}).  An error will be signaled 
in the case in which a module with the same name has already been 
loaded, if that module specifies a version and that version is not 
compatible with @var{version-spec}.

Signal error if module name is not resolvable.
@end deffn


@c FIXME::martin: Is this correct, and is there more to say?
@c FIXME::martin: Define term and concept `syntax transformer' somewhere.

@deffn syntax use-syntax module-name
Load the module @code{module-name} and use its syntax
transformer as the syntax transformer for the currently defined module,
as well as installing it as the current syntax transformer.
@end deffn

@deffn syntax @@ module-name binding-name
Refer to the binding named @var{binding-name} in module
@var{module-name}.  The binding must have been exported by the module.
@end deffn

@deffn syntax @@@@ module-name binding-name
Refer to the binding named @var{binding-name} in module
@var{module-name}.  The binding must not have been exported by the
module.  This syntax is only intended for debugging purposes or as a
last resort.
@end deffn

@node Creating Guile Modules
@subsection Creating Guile Modules

When you want to create your own modules, you have to take the following
steps:

@itemize @bullet
@item
Create a Scheme source file and add all variables and procedures you wish
to export, or which are required by the exported procedures.

@item
Add a @code{define-module} form at the beginning.

@item
Export all bindings which should be in the public interface, either
by using @code{define-public} or @code{export} (both documented below).
@end itemize

@c begin (scm-doc-string "boot-9.scm" "define-module")
@deffn syntax define-module module-name [options @dots{}]
@var{module-name} is of the form @code{(hierarchy file)}.  One
example of this is

@lisp
(define-module (ice-9 popen))
@end lisp

@code{define-module} makes this module available to Guile programs under
the given @var{module-name}.

The @var{options} are keyword/value pairs which specify more about the
defined module.  The recognized options and their meaning is shown in
the following table.

@c fixme: Should we use "#:" or ":"?

@table @code
@item #:use-module @var{interface-specification}
Equivalent to a @code{(use-modules @var{interface-specification})}
(@pxref{Using Guile Modules}).

@item #:use-syntax @var{module}
Use @var{module} when loading the currently defined module, and install
it as the syntax transformer.

@item #:autoload @var{module} @var{symbol-list}
@cindex autoload
Load @var{module} when any of @var{symbol-list} are accessed.  For
example,

@example
(define-module (my mod)
  #:autoload (srfi srfi-1) (partition delete-duplicates))
...
(if something
    (set! foo (delete-duplicates ...)))
@end example

When a module is autoloaded, all its bindings become available.
@var{symbol-list} is just those that will first trigger the load.

An autoload is a good way to put off loading a big module until it's
really needed, for instance for faster startup or if it will only be
needed in certain circumstances.

@code{@@} can do a similar thing (@pxref{Using Guile Modules}), but in
that case an @code{@@} form must be written every time a binding from
the module is used.

@item #:export @var{list}
@cindex export
Export all identifiers in @var{list} which must be a list of symbols
or pairs of symbols. This is equivalent to @code{(export @var{list})} 
in the module body.

@item #:re-export @var{list}
@cindex re-export
Re-export all identifiers in @var{list} which must be a list of
symbols or pairs of symbols.  The symbols in @var{list} must be 
imported by the current module from other modules.  This is equivalent
to @code{re-export} below.

@item #:export-syntax @var{list}
@cindex export-syntax
Export all identifiers in @var{list} which must be a list of symbols
or pairs of symbols.  The identifiers in @var{list} must refer to 
macros (@pxref{Macros}) defined in the current module.  This is 
equivalent to @code{(export-syntax @var{list})} in the module body.

@item #:re-export-syntax @var{list}
@cindex re-export-syntax
Re-export all identifiers in @var{list} which must be a list of
symbols or pairs of symbols.  The symbols in @var{list} must refer to
macros imported by the current module from other modules.  This is 
equivalent to @code{(re-export-syntax @var{list})} in the module body. 

@item #:replace @var{list}
@cindex replace
@cindex replacing binding
@cindex overriding binding
@cindex duplicate binding
Export all identifiers in @var{list} (a list of symbols or pairs of
symbols) and mark them as @dfn{replacing bindings}.  In the module 
user's name space, this will have the effect of replacing any binding 
with the same name that is not also ``replacing''.  Normally a 
replacement results in an ``override'' warning message, 
@code{#:replace} avoids that.

In general, a module that exports a binding for which the @code{(guile)}
module already has a definition should use @code{#:replace} instead of
@code{#:export}.  @code{#:replace}, in a sense, lets Guile know that the
module @emph{purposefully} replaces a core binding.  It is important to
note, however, that this binding replacement is confined to the name
space of the module user.  In other words, the value of the core binding
in question remains unchanged for other modules.

Note that although it is often a good idea for the replaced binding to
remain compatible with a binding in @code{(guile)}, to avoid surprising
the user, sometimes the bindings will be incompatible.  For example,
SRFI-19 exports its own version of @code{current-time} (@pxref{SRFI-19
Time}) which is not compatible with the core @code{current-time}
function (@pxref{Time}).  Guile assumes that a user importing a module
knows what she is doing, and uses @code{#:replace} for this binding
rather than @code{#:export}.

The @code{#:duplicates} (see below) provides fine-grain control about
duplicate binding handling on the module-user side.

@item #:version @var{list}
@cindex module version
Specify a version for the module in the form of @var{list}, a list of
zero or more exact, nonnegative integers.  The corresponding 
@code{#:version} option in the @code{use-modules} form allows callers
to restrict the value of this option in various ways.

@item #:duplicates @var{list}
@cindex duplicate binding handlers
@cindex duplicate binding
@cindex overriding binding
Tell Guile to handle duplicate bindings for the bindings imported by
the current module according to the policy defined by @var{list}, a
list of symbols.  @var{list} must contain symbols representing a
duplicate binding handling policy chosen among the following:

@table @code
@item check
Raises an error when a binding is imported from more than one place.
@item warn
Issue a warning when a binding is imported from more than one place
and leave the responsibility of actually handling the duplication to
the next duplicate binding handler.
@item replace
When a new binding is imported that has the same name as a previously
imported binding, then do the following:

@enumerate
@item
@cindex replacing binding
If the old binding was said to be @dfn{replacing} (via the
@code{#:replace} option above) and the new binding is not replacing,
the keep the old binding.
@item
If the old binding was not said to be replacing and the new binding is
replacing, then replace the old binding with the new one.
@item
If neither the old nor the new binding is replacing, then keep the old
one.
@end enumerate

@item warn-override-core
Issue a warning when a core binding is being overwritten and actually
override the core binding with the new one.
@item first
In case of duplicate bindings, the firstly imported binding is always
the one which is kept.
@item last
In case of duplicate bindings, the lastly imported binding is always
the one which is kept.
@item noop
In case of duplicate bindings, leave the responsibility to the next
duplicate handler.
@end table

If @var{list} contains more than one symbol, then the duplicate
binding handlers which appear first will be used first when resolving
a duplicate binding situation.  As mentioned above, some resolution
policies may explicitly leave the responsibility of handling the
duplication to the next handler in @var{list}.

@findex default-duplicate-binding-handler
The default duplicate binding resolution policy is given by the
@code{default-duplicate-binding-handler} procedure, and is

@lisp
(replace warn-override-core warn last)
@end lisp

@item #:no-backtrace
@cindex no backtrace
Tell Guile not to record information for procedure backtraces when
executing the procedures in this module.

@item #:pure
@cindex pure module
Create a @dfn{pure} module, that is a module which does not contain any
of the standard procedure bindings except for the syntax forms.  This is
useful if you want to create @dfn{safe} modules, that is modules which
do not know anything about dangerous procedures.
@end table

@end deffn
@c end

@deffn syntax export variable @dots{}
Add all @var{variable}s (which must be symbols or pairs of symbols) to 
the list of exported bindings of the current module.  If @var{variable}
is a pair, its @code{car} gives the name of the variable as seen by the
current module and its @code{cdr} specifies a name for the binding in
the current module's public interface.
@end deffn

@c begin (scm-doc-string "boot-9.scm" "define-public")
@deffn syntax define-public @dots{}
Equivalent to @code{(begin (define foo ...) (export foo))}.
@end deffn
@c end

@deffn syntax re-export variable @dots{}
Add all @var{variable}s (which must be symbols or pairs of symbols) to 
the list of re-exported bindings of the current module.  Pairs of 
symbols are handled as in @code{export}.  Re-exported bindings must be
imported by the current module from some other module.
@end deffn

@node Module System Reflection
@subsection Module System Reflection

The previous sections have described a declarative view of the module
system.  You can also work with it programmatically by accessing and
modifying various parts of the Scheme objects that Guile uses to
implement the module system.

At any time, there is a @dfn{current module}.  This module is the one
where a top-level @code{define} and similar syntax will add new
bindings.  You can find other module objects with @code{resolve-module},
for example.

These module objects can be used as the second argument to @code{eval}.

@deffn {Scheme Procedure} current-module
Return the current module object.
@end deffn

@deffn {Scheme Procedure} set-current-module module
Set the current module to @var{module} and return
the previous current module.
@end deffn

@deffn {Scheme Procedure} save-module-excursion thunk
Call @var{thunk} within a @code{dynamic-wind} such that the module that
is current at invocation time is restored when @var{thunk}'s dynamic
extent is left (@pxref{Dynamic Wind}).

More precisely, if @var{thunk} escapes non-locally, the current module
(at the time of escape) is saved, and the original current module (at
the time @var{thunk}'s dynamic extent was last entered) is restored.  If
@var{thunk}'s dynamic extent is re-entered, then the current module is
saved, and the previously saved inner module is set current again.
@end deffn

@deffn {Scheme Procedure} resolve-module name
Find the module named @var{name} and return it.  When it has not already
been defined, try to auto-load it.  When it can't be found that way
either, create an empty module.  The name is a list of symbols.
@end deffn

@deffn {Scheme Procedure} resolve-interface name
Find the module named @var{name} as with @code{resolve-module} and
return its interface.  The interface of a module is also a module
object, but it contains only the exported bindings.
@end deffn

@deffn {Scheme Procedure} module-use! module interface
Add @var{interface} to the front of the use-list of @var{module}.  Both
arguments should be module objects, and @var{interface} should very
likely be a module returned by @code{resolve-interface}.
@end deffn


@node Included Guile Modules
@subsection Included Guile Modules

@c FIXME::martin: Review me!

Some modules are included in the Guile distribution; here are references
to the entries in this manual which describe them in more detail:

@table @strong
@item boot-9
boot-9 is Guile's initialization module, and it is always loaded when
Guile starts up.

@item (ice-9 debug)
Mikael Djurfeldt's source-level debugging support for Guile
(@pxref{Tracing}).

@item (ice-9 expect)
Actions based on matching input from a port (@pxref{Expect}).

@item (ice-9 format)
Formatted output in the style of Common Lisp (@pxref{Formatted
Output}).

@item (ice-9 ftw)
File tree walker (@pxref{File Tree Walk}).

@item (ice-9 getopt-long)
Command line option processing (@pxref{getopt-long}).

@item (ice-9 history)
Refer to previous interactive expressions (@pxref{Value History}).

@item (ice-9 popen)
Pipes to and from child processes (@pxref{Pipes}).

@item (ice-9 pretty-print)
Nicely formatted output of Scheme expressions and objects
(@pxref{Pretty Printing}).

@item (ice-9 q)
First-in first-out queues (@pxref{Queues}).

@item (ice-9 rdelim)
Line- and character-delimited input (@pxref{Line/Delimited}).

@item (ice-9 readline)
@code{readline} interactive command line editing (@pxref{Readline
Support}).

@item (ice-9 receive)
Multiple-value handling with @code{receive} (@pxref{Multiple Values}).

@item (ice-9 regex)
Regular expression matching (@pxref{Regular Expressions}).

@item (ice-9 rw)
Block string input/output (@pxref{Block Reading and Writing}).

@item (ice-9 streams)
Sequence of values calculated on-demand (@pxref{Streams}).

@item (ice-9 syncase)
R5RS @code{syntax-rules} macro system (@pxref{Syntax Rules}).

@item (ice-9 threads)
Guile's support for multi threaded execution (@pxref{Scheduling}).

@item (ice-9 documentation)
Online documentation (REFFIXME).

@item (srfi srfi-1)
A library providing a lot of useful list and pair processing
procedures (@pxref{SRFI-1}).

@item (srfi srfi-2)
Support for @code{and-let*} (@pxref{SRFI-2}).

@item (srfi srfi-4)
Support for homogeneous numeric vectors (@pxref{SRFI-4}).

@item (srfi srfi-6)
Support for some additional string port procedures (@pxref{SRFI-6}).

@item (srfi srfi-8)
Multiple-value handling with @code{receive} (@pxref{SRFI-8}).

@item (srfi srfi-9)
Record definition with @code{define-record-type} (@pxref{SRFI-9}).

@item (srfi srfi-10)
Read hash extension @code{#,()} (@pxref{SRFI-10}).

@item (srfi srfi-11)
Multiple-value handling with @code{let-values} and @code{let*-values}
(@pxref{SRFI-11}).

@item (srfi srfi-13)
String library (@pxref{SRFI-13}).

@item (srfi srfi-14)
Character-set library (@pxref{SRFI-14}).

@item (srfi srfi-16)
@code{case-lambda} procedures of variable arity (@pxref{SRFI-16}).

@item (srfi srfi-17)
Getter-with-setter support (@pxref{SRFI-17}).

@item (srfi srfi-19)
Time/Date library (@pxref{SRFI-19}).

@item (srfi srfi-26)
Convenient syntax for partial application (@pxref{SRFI-26})

@item (srfi srfi-31)
@code{rec} convenient recursive expressions (@pxref{SRFI-31})

@item (ice-9 slib)
This module contains hooks for using Aubrey Jaffer's portable Scheme
library SLIB from Guile (@pxref{SLIB}).
@end table


@node R6RS Version References
@subsection R6RS Version References

Guile's module system includes support for locating modules based on
a declared version specifier of the same form as the one described in
R6RS (@pxref{Library form, R6RS Library Form,, r6rs, The Revised^6 
Report on the Algorithmic Language Scheme}).  By using the 
@code{#:version} keyword in a @code{define-module} form, a module may
specify a version as a list of zero or more exact, nonnegative integers.

This version can then be used to locate the module during the module
search process.  Client modules and callers of the @code{use-modules} 
function may specify constraints on the versions of target modules by
providing a @dfn{version reference}, which has one of the following
forms:

@lisp
 (@var{sub-version-reference} ...)
 (and @var{version-reference} ...)
 (or @var{version-reference} ...)
 (not @var{version-reference})
@end lisp

in which @var{sub-version-reference} is in turn one of:

@lisp
 (@var{sub-version})
 (>= @var{sub-version})
 (<= @var{sub-version})
 (and @var{sub-version-reference} ...)
 (or @var{sub-version-reference} ...)
 (not @var{sub-version-reference})
@end lisp

in which @var{sub-version} is an exact, nonnegative integer as above. A
version reference matches a declared module version if each element of
the version reference matches a corresponding element of the module 
version, according to the following rules:

@itemize @bullet
@item
The @code{and} sub-form matches a version or version element if every 
element in the tail of the sub-form matches the specified version or 
version element.

@item
The @code{or} sub-form matches a version or version element if any 
element in the tail of the sub-form matches the specified version or
version element.

@item
The @code{not} sub-form matches a version or version element if the tail
of the sub-form does not match the version or version element.  

@item
The @code{>=} sub-form matches a version element if the element is 
greater than or equal to the @var{sub-version} in the tail of the 
sub-form.

@item
The @code{<=} sub-form matches a version element if the version is less
than or equal to the @var{sub-version} in the tail of the sub-form.

@item
A @var{sub-version} matches a version element if one is @var{eqv?} to
the other.
@end itemize

For example, a module declared as:

@lisp
 (define-module (mylib mymodule) #:version (1 2 0))
@end lisp

would be successfully loaded by any of the following @code{use-modules}
expressions:

@lisp
 (use-modules ((mylib mymodule) #:version (1 2 (>= 0))))
 (use-modules ((mylib mymodule) #:version (or (1 2 0) (1 2 1))))
 (use-modules ((mylib mymodule) #:version ((and (>= 1) (not 2)) 2 0)))
@end lisp


@node R6RS Libraries
@subsection R6RS Libraries

In addition to the API described in the previous sections, you also
have the option to create modules using the portable @code{library} form
described in R6RS (@pxref{Library form, R6RS Library Form,, r6rs, The
Revised^6 Report on the Algorithmic Language Scheme}), and to import 
libraries created in this format by other programmers.  Guile's R6RS 
library implementation takes advantage of the flexibility built into the
module system by expanding the R6RS library form into a corresponding 
Guile @code{define-module} form that specifies equivalent import and 
export requirements and includes the same body expressions.  The library
expression:

@lisp
  (library (mylib (1 2))
    (import (otherlib (3)))
    (export mybinding))
@end lisp

is equivalent to the module definition:

@lisp
  (define-module (mylib)
    #:version (1 2)
    #:use-module ((otherlib) #:version (3))
    #:export (mybinding))
@end lisp

Central to the mechanics of R6RS libraries is the concept of import
and export @dfn{levels}, which control the visibility of bindings at
various phases of a library's lifecycle --- macros necessary to 
expand forms in the library's body need to be available at expand 
time; variables used in the body of a procedure exported by the
library must be available at runtime.  R6RS specifies the optional
@code{for} sub-form of an @emph{import set} specification (see below)
as a mechanism by which a library author can indicate that a
particular library import should take place at a particular phase 
with respect to the lifecycle of the importing library.  

Guile's libraries implementation uses a technique called 
@dfn{implicit phasing} (first described by Abdulaziz Ghuloum and R. 
Kent Dybvig), which allows the expander and compiler to automatically 
determine the necessary visibility of a binding imported from another 
library.  As such, the @code{for} sub-form described below is ignored by
Guile (but may be required by Schemes in which phasing is explicit).

@deffn {Scheme Syntax} library name (export export-spec ...) (import import-spec ...) body ...
Defines a new library with the specified name, exports, and imports,
and evaluates the specified body expressions in this library's 
environment.

The library @var{name} is a non-empty list of identifiers, optionally
ending with a version specification of the form described above
(@pxref{Creating Guile Modules}).

Each @var{export-spec} is the name of a variable defined or imported
by the library, or must take the form 
@code{(rename (internal-name external-name) ...)}, where the 
identifier @var{internal-name} names a variable defined or imported 
by the library and @var{external-name} is the name by which the
variable is seen by importing libraries.

Each @var{import-spec} must be either an @dfn{import set} (see below)
or must be of the form @code{(for import-set import-level ...)}, 
where each @var{import-level} is one of:

@lisp
  run
  expand
  (meta @var{level})
@end lisp

where @var{level} is an integer.  Note that since Guile does not
require explicit phase specification, any @var{import-set}s found 
inside of @code{for} sub-forms will be ``unwrapped'' during 
expansion and processed as if they had been specified directly.

Import sets in turn take one of the following forms:

@lisp
  @var{library-reference}
  (library @var{library-reference})
  (only @var{import-set} @var{identifier} ...)
  (except @var{import-set} @var{identifier} ...)
  (prefix @var{import-set} @var{identifier})
  (rename @var{import-set} (@var{internal-identifier} @var{external-identifier}) ...)
@end lisp

where @var{library-reference} is a non-empty list of identifiers
ending with an optional version reference (@pxref{R6RS Version 
References}), and the other sub-forms have the following semantics,
defined recursively on nested @var{import-set}s:

@itemize @bullet

@item
The @code{library} sub-form is used to specify libraries for import
whose names begin with the identifier ``library.''

@item
The @code{only} sub-form imports only the specified @var{identifier}s
from the given @var{import-set}.

@item
The @code{except} sub-form imports all of the bindings exported by 
@var{import-set} except for those that appear in the specified list
of @var{identifier}s.

@item
The @code{prefix} sub-form imports all of the bindings exported
by @var{import-set}, first prefixing them with the specified
@var{identifier}.

@item
The @code{rename} sub-form imports all of the identifiers exported
by @var{import-set}.  The binding for each @var{internal-identifier}
among these identifiers is made visible to the importing library as
the corresponding @var{external-identifier}; all other bindings are
imported using the names provided by @var{import-set}.

@end itemize

Note that because Guile translates R6RS libraries into module 
definitions, an import specification may be used to declare a
dependency on a native Guile module --- although doing so may make 
your libraries less portable to other Schemes.

@end deffn

@deffn {Scheme Syntax} import import-spec ...
Import into the current environment the libraries specified by the
given import specifications, where each @var{import-spec} takes the
same form as in the @code{library} form described above.
@end deffn


@node Accessing Modules from C
@subsection Accessing Modules from C

The last sections have described how modules are used in Scheme code,
which is the recommended way of creating and accessing modules.  You
can also work with modules from C, but it is more cumbersome.

The following procedures are available.

@deftypefn {C Procedure} SCM scm_current_module ()
Return the module that is the @emph{current module}.
@end deftypefn

@deftypefn {C Procedure} SCM scm_set_current_module (SCM @var{module})
Set the current module to @var{module} and return the previous current
module.
@end deftypefn

@deftypefn {C Procedure} SCM scm_c_call_with_current_module (SCM @var{module}, SCM (*@var{func})(void *), void *@var{data})
Call @var{func} and make @var{module} the current module during the
call.  The argument @var{data} is passed to @var{func}.  The return
value of @code{scm_c_call_with_current_module} is the return value of
@var{func}.
@end deftypefn

@deftypefn {C Procedure} SCM scm_c_lookup (const char *@var{name})
Return the variable bound to the symbol indicated by @var{name} in the
current module.  If there is no such binding or the symbol is not
bound to a variable, signal an error.
@end deftypefn

@deftypefn {C Procedure} SCM scm_lookup (SCM @var{name})
Like @code{scm_c_lookup}, but the symbol is specified directly.
@end deftypefn

@deftypefn {C Procedure} SCM scm_c_module_lookup (SCM @var{module}, const char *@var{name})
@deftypefnx {C Procedure} SCM scm_module_lookup (SCM @var{module}, SCM @var{name})
Like @code{scm_c_lookup} and @code{scm_lookup}, but the specified
module is used instead of the current one.
@end deftypefn

@deftypefn {C Procedure} SCM scm_c_define (const char *@var{name}, SCM @var{val})
Bind the symbol indicated by @var{name} to a variable in the current
module and set that variable to @var{val}.  When @var{name} is already
bound to a variable, use that.  Else create a new variable.
@end deftypefn

@deftypefn {C Procedure} SCM scm_define (SCM @var{name}, SCM @var{val})
Like @code{scm_c_define}, but the symbol is specified directly.
@end deftypefn

@deftypefn {C Procedure} SCM scm_c_module_define (SCM @var{module}, const char *@var{name}, SCM @var{val})
@deftypefnx {C Procedure} SCM scm_module_define (SCM @var{module}, SCM @var{name}, SCM @var{val})
Like @code{scm_c_define} and @code{scm_define}, but the specified
module is used instead of the current one.
@end deftypefn

@deftypefn {C Procedure} SCM scm_module_reverse_lookup (SCM @var{module}, SCM @var{variable})
Find the symbol that is bound to @var{variable} in @var{module}.  When no such binding is found, return @var{#f}.
@end deftypefn

@deftypefn {C Procedure} SCM scm_c_define_module (const char *@var{name}, void (*@var{init})(void *), void *@var{data})
Define a new module named @var{name} and make it current while
@var{init} is called, passing it @var{data}.  Return the module.

The parameter @var{name} is a string with the symbols that make up
the module name, separated by spaces.  For example, @samp{"foo bar"} names
the module @samp{(foo bar)}.

When there already exists a module named @var{name}, it is used
unchanged, otherwise, an empty module is created.
@end deftypefn

@deftypefn {C Procedure} SCM scm_c_resolve_module (const char *@var{name})
Find the module name @var{name} and return it.  When it has not
already been defined, try to auto-load it.  When it can't be found
that way either, create an empty module.  The name is interpreted as
for @code{scm_c_define_module}.
@end deftypefn

@deftypefn {C Procedure} SCM scm_resolve_module (SCM @var{name})
Like @code{scm_c_resolve_module}, but the name is given as a real list
of symbols.
@end deftypefn

@deftypefn {C Procedure} SCM scm_c_use_module (const char *@var{name})
Add the module named @var{name} to the uses list of the current
module, as with @code{(use-modules @var{name})}.  The name is
interpreted as for @code{scm_c_define_module}.
@end deftypefn

@deftypefn {C Procedure} SCM scm_c_export (const char *@var{name}, ...)
Add the bindings designated by @var{name}, ... to the public interface
of the current module.  The list of names is terminated by
@code{NULL}.
@end deftypefn


@node Variables
@subsection Variables
@tpindex Variables

Each module has its own hash table, sometimes known as an @dfn{obarray},
that maps the names defined in that module to their corresponding
variable objects.

A variable is a box-like object that can hold any Scheme value.  It is
said to be @dfn{undefined} if its box holds a special Scheme value that
denotes undefined-ness (which is different from all other Scheme values,
including for example @code{#f}); otherwise the variable is
@dfn{defined}.

On its own, a variable object is anonymous.  A variable is said to be
@dfn{bound} when it is associated with a name in some way, usually a
symbol in a module obarray.  When this happens, the relationship is
mutual: the variable is bound to the name (in that module), and the name
(in that module) is bound to the variable.

(That's the theory, anyway.  In practice, defined-ness and bound-ness
sometimes get confused, because Lisp and Scheme implementations have
often conflated --- or deliberately drawn no distinction between --- a
name that is unbound and a name that is bound to a variable whose value
is undefined.  We will try to be clear about the difference and explain
any confusion where it is unavoidable.)

Variables do not have a read syntax.  Most commonly they are created and
bound implicitly by @code{define} expressions: a top-level @code{define}
expression of the form

@lisp
(define @var{name} @var{value})
@end lisp

@noindent
creates a variable with initial value @var{value} and binds it to the
name @var{name} in the current module.  But they can also be created
dynamically by calling one of the constructor procedures
@code{make-variable} and @code{make-undefined-variable}.

@deffn {Scheme Procedure} make-undefined-variable
@deffnx {C Function} scm_make_undefined_variable ()
Return a variable that is initially unbound.
@end deffn

@deffn {Scheme Procedure} make-variable init
@deffnx {C Function} scm_make_variable (init)
Return a variable initialized to value @var{init}.
@end deffn

@deffn {Scheme Procedure} variable-bound? var
@deffnx {C Function} scm_variable_bound_p (var)
Return @code{#t} iff @var{var} is bound to a value.
Throws an error if @var{var} is not a variable object.
@end deffn

@deffn {Scheme Procedure} variable-ref var
@deffnx {C Function} scm_variable_ref (var)
Dereference @var{var} and return its value.
@var{var} must be a variable object; see @code{make-variable}
and @code{make-undefined-variable}.
@end deffn

@deffn {Scheme Procedure} variable-set! var val
@deffnx {C Function} scm_variable_set_x (var, val)
Set the value of the variable @var{var} to @var{val}.
@var{var} must be a variable object, @var{val} can be any
value. Return an unspecified value.
@end deffn

@deffn {Scheme Procedure} variable? obj
@deffnx {C Function} scm_variable_p (obj)
Return @code{#t} iff @var{obj} is a variable object, else
return @code{#f}.
@end deffn


@node provide and require
@subsection provide and require

Aubrey Jaffer, mostly to support his portable Scheme library SLIB,
implemented a provide/require mechanism for many Scheme implementations.
Library files in SLIB @emph{provide} a feature, and when user programs
@emph{require} that feature, the library file is loaded in.

For example, the file @file{random.scm} in the SLIB package contains the
line

@lisp
(provide 'random)
@end lisp

so to use its procedures, a user would type

@lisp
(require 'random)
@end lisp

and they would magically become available, @emph{but still have the same
names!}  So this method is nice, but not as good as a full-featured
module system.

When SLIB is used with Guile, provide and require can be used to access
its facilities.

@node Environments
@subsection Environments
@cindex environment

Scheme, as defined in R5RS, does @emph{not} have a full module system.
However it does define the concept of a top-level @dfn{environment}.
Such an environment maps identifiers (symbols) to Scheme objects such
as procedures and lists: @ref{About Closure}.  In other words, it
implements a set of @dfn{bindings}.

Environments in R5RS can be passed as the second argument to
@code{eval} (@pxref{Fly Evaluation}).  Three procedures are defined to
return environments: @code{scheme-report-environment},
@code{null-environment} and @code{interaction-environment} (@pxref{Fly
Evaluation}).

In addition, in Guile any module can be used as an R5RS environment,
i.e., passed as the second argument to @code{eval}.

Note: the following two procedures are available only when the 
@code{(ice-9 r5rs)} module is loaded:

@lisp
(use-modules (ice-9 r5rs))
@end lisp

@deffn {Scheme Procedure} scheme-report-environment version
@deffnx {Scheme Procedure} null-environment version
@var{version} must be the exact integer `5', corresponding to revision
5 of the Scheme report (the Revised^5 Report on Scheme).
@code{scheme-report-environment} returns a specifier for an
environment that is empty except for all bindings defined in the
report that are either required or both optional and supported by the
implementation. @code{null-environment} returns a specifier for an
environment that is empty except for the (syntactic) bindings for all
syntactic keywords defined in the report that are either required or
both optional and supported by the implementation.

Currently Guile does not support values of @var{version} for other
revisions of the report.

The effect of assigning (through the use of @code{eval}) a variable
bound in a @code{scheme-report-environment} (for example @code{car})
is unspecified.  Currently the environments specified by
@code{scheme-report-environment} are not immutable in Guile.
@end deffn



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